Seven Spark Plug Myths Busted – UnderhoodService
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Seven Spark Plug Myths Busted

If it fits, it will work. Spark plugs can be changed when the engine is hot. The thicker the wire means more of the fire! … False, all false. These are all spark plug myths. We bust these and a few others in this month’s look at spark plugs.

If it fits, it will work. 
False. Even if a spark plug fits in the hole in the head and the correct heat range, the electrodes may not be designed for that ­specific engine. The wrong plug electrode design can affect the flame front inside the combustion chamber and even increase the compression ratio. Always consult a catalog before using an old cross-reference chart. 
Spark plugs can be changed when the engine is hot.
Changing the plugs while the engine is hot is a big ­no-no. Even removing the plugs while the engine is hot can damage the plugs and the head due to differing rates of thermal expansion. Always wait until the engine is cold. 

The thicker the wire means more of the fire!
False. The actual core of a spark plug wire, which carries the spark, is small and its resistance is specified by the OEM. The girth of a spark plug wire is the insulation and shielding. Performance wires that are thicker can offer greater electromagnetic shielding for ­running performance ignition boxes. Racing plug wires offer more thermal protection and ­abrasion resistance.
The color matters.
Yes and No. A plug that has racked up the miles should have a brownish or greyish color on the insulator and electrodes, but some gas additives have been known to turn fresh insulators blue, pink or purple. Pay more attention to the deposits and where they occur to determine if there is a problem. 
Measure, gap and grind.
Never on modern applications. Modern precious metal, fine wire and tip-to-tip plugs should be handled with extreme care. You should never try to change the gap of the plug unless the manufacturer advises it. Trying to measure or clean the plug will cause damage and ­misfires.     
You need to put anti-seize on the threads.
False. Most spark plugs come from the factory with a coating or plating that is designed to prevent galling of the threads. Applying a coating of anti-seize will only lead to incorrect torque readings. Use of anti-seize can increase torque values by 20 percent. 
Corona stains are a sign of an incorrect heat range or blowby.
No. Corona stains on the exposed ceramic body are normal. Corona stains are caused by the high amount of static electricity attracting particles of oil and dirt to the body of the plug — not blowby gases or thermal distress.

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